11 - Self-healing composites for aerospace...

Self-healing composites foraerospace applications 11R. Das 1, C. Melchior 2, K.M. Karumbaiah 1

1University of Auckland, Auckland, New Zealand; 2National Polytechnic Institute ofChemical Engineering and Technology (INP-ENSIACET), Toulouse, France

11.1 Introduction

Metal alloys are being replaced by composite materials in several advanced applica-tions, such as aerospace, automotive, marine and building components, owing to theirlight weight and high mechanical properties. Hence, developing damage-resistant anddurable composite materials is necessary. Certainly, fibreematrix de-bonding, matrixmicrocracking and impact damage are major failure modes routinely encountered inthe applications of composite materials. Furthermore, deployment and maintenanceof composite materials pose a challenge for critical structural parts, such as wingsand fins. Hence, advanced materials and methodologies are essential to address theseproblems. Self-healing technology using composite materials seems to be promising,as it is designed to heal or repair fracture and damage initiation and/or propagation instructures. Self-healing composite materials prevent failure and extend the lifetime ofcritical structures. Maintenance of structures can be considerably simplified because ofthese materials, which can trigger an almost auto-repair with some of them notrequiring any external intervention to start the healing process.

Self-healing composite materials are capable of auto-repairing upon initiation ofdamage. The early development concept of healing ability relied on mimicking livingorganisms, like trees and animals, which motivated research in developing self-healingmaterials. Self-healing materials and composites have been studied for the pastfew decades, specifically fuelled by the development of self-healing epoxy resin(White et al., 2002).

Self-healing mechanisms can be divided into two types, extrinsic and intrinsic heal-ing. Extrinsic healing is based on the use of a healing agent as an additional additive,whereas intrinsic healing involves a reversible molecular bond (supramolecular chem-istry) in the structure of the material. Additionally, classification can also be madebased on the healing method, whether autonomic healing or non-autonomic healing(i.e., with or without external stimulus). Some of the well-known methods of devel-oping self-healing composite materials are inclusions of microcapsules, hollow fibresor a vascular network containing healing agents (Blaiszik et al., 2008). Self-healingcan also be thermally activated, using reversible interactions or dissolved thermoplasticpolymers. The shape memory effect has also been used to demonstrate self-healingproperties.

Advanced Composite Materials for Aerospace Engineering. http://dx.doi.org/10.1016/B978-0-08-100037-3.00011-0Copyright © 2016 Elsevier Ltd. All rights reserved.

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Self-healing composite materials include polymer matrix composites, ceramic ma-trix composites (CMCs), metal matrix composites (MMCs) and cementitious compos-ites. Although research on self-healing materials is relatively new, a few commercialself-healing materials, like the Reverlink™ elastomer, are manufactured by Arkema(Cordier et al., 2008), and there are many potential applications of self-healingcomposites (e.g., resistant fabrics, resealing tires and long-life batteries). Self-healingcoatings for corrosion protection or barrier protection have found early commercialapplications.

One primary area where self-healing composites may have strong potential andalready have found crucial roles is the aerospace sector. In the aerospace industry,self-healing materials have the ability to repair damage that may have occurred duringa flight and increase the lifetime of the components. One key advantage of self-healingcomposite materials is to repair dynamic damage and maintain impact resistance.Self-healing composites can be used in various capacities (e.g., in aerospace structuralparts to prevent damage and increase lifetime and also in anticorrosion and barriercoatings).

In this chapter, the primary focus is the self-healing composites that can be used inaerospace applications. Different composite materials, their manufacturing techniques,different types of self-healing concepts and various applications are presented ingeneral and also specifically in the context of the aerospace sector. The self-healingcomposites can be polymer, ceramic or metal matrix composites. Initially, differentself-healing concepts are explained, followed by the classification of self-healing com-posites. Self-healing approaches, such as microcapsules, hollow fibres, vascularnetwork and intrinsic healing, are described in the subsequent sections. Different typesof self-healing composites, such as polymer matrix composites (E-glasseepoxy andcarbon fibreeepoxy), CMCs and MMCs, are then discussed. Various properties,like mechanical, corrosion and barrier protection properties, influenced by theself-healing ability of composites are briefed. Finally, applications of self-healingcomposites, especially in the aerospace industry, are discussed.

11.2 Self-healing concept

A self-healing material has the ability to auto-repair under a ‘stimulus’ when fractureor damage occurs during in-service operation either accidently or due to ageing.The ‘self-healing’ concept is inspired by biological systems or living beings and hasbeen widely developed during the last decade (White et al., 2002). Additionally,a limited number of commercial applications, like the self-healing elastomerReverlink™ (Cordier et al., 2008), have been found to date.

The most general classification of self-healing materials is based on extrinsic orintrinsic healing (Garcia and Fischer, 2014). Other forms of classification are basedon autonomic healing and nonautonomic healing (Blaiszik et al., 2010). An autonomicself-healing material does not require any external trigger to initiate the self-healing

334 Advanced Composite Materials for Aerospace Engineering

process, whereas nonautonomic materials need an external trigger, such as heat orpressure, to heal. Commonly, self-healing approaches include the use of healing agentscontained in microcapsules (White et al., 2002), hollow fibres (Pang and Bond, 2005),a mesoporous network (Hamilton et al., 2010) or dissolved thermoplastics. Differencesbetween intrinsic and extrinsic self-healing approaches are discussed in the followingsections.

11.2.1 Extrinsic healing

The extrinsic healing process is based on the use of a healing agent contained in thematrix as a separate phase (White et al., 2002). The healing agent is usually in theliquid state, placed in the form of microcapsules or hollow fibres. In most approaches,the healing agent is used with a catalyst, which can also be encapsulated or dissolved inthe matrix. When damage occurs, local containers are broken, and the healing agentand catalyst are released, resulting in the healing of cracks, preventing crack growthand fracture failure of the structure.

The main extrinsic healing approaches are:

1. Healing agents are contained in the form of microcapsules, and catalysts are dispersed in thematrix. In a few cases, the healing agent can also react itself; in those cases, there is no needfor a catalyst to initiate the process (White et al., 2002).

2. The healing agent is contained in the form of tubes, which is essentially the same as the mi-crocapsules, and only the shape of the container is varied (Pang and Bond, 2005).

3. The mesoporous network contains healing agents or delivers them from an external reservoir,in case of damage (Hamilton et al., 2010).

Fig. 11.1 shows a polymer composite with an encapsulated healing agent anddispersed catalyst. When the material is damaged, the healing agent flows into thecrack and repairs the crack with the aid of the catalyst. In the case of hollow fibres,the healing agent is delivered or released into the cracks when damage has occurred.The extrinsic healing concept is based on the response after or at the onset of damage.Furthermore, materials using extrinsic approaches are vulnerable to repeated damageat the same location. Also, healing of structures is not possible once the healing agentsare used up or containers become empty. Current research focusses on improvement ofhealing agents and catalysts, and on new encapsulation techniques that can reactwithout a catalyst when released.

11.2.2 Intrinsic healing

Intrinsic healing is based on specific properties of certain materials, such as molecularstructures, chemical or physical bonds. In most cases, intrinsic self-healing requiresexternal stimuli (e.g., high temperatures). But there are few intrinsic self-healing ma-terials which do not require any external stimuli to initiate the process of healing. Threedifferent modes of intrinsic self-healing have been proposed (Zhang and Rong, 2012).

Self-healing composites for aerospace applications 335

Intrinsic self-healing can be achieved by reversible covalent bonds, thermoreversiblephysical interactions or supramolecular chemistry.

1. Reversible covalent chemistry implies covalent bonds that can dissociate and reassociate un-der damage. Such reactions mostly include ring-chain equilibrium. One widely studiedexample is the retro DielseAlder (rDA) reaction (Fig. 11.2) (Park et al., 2009).

2. Research on thermoreversible physical interactions is extensive and has mainly focussed onionomers (Varley and van der Zwaag, 2008; García et al., 2011).

3. Supramolecular chemistry is promising and has been one of the first commercial applicationsof self-healing polymers (Cordier et al., 2008). Reversible supramolecular interactions arelow-energy interactions that have an influence on the overall properties of a material, ifwell designed. Possible avenues of obtaining these interactions are based on hydrogenbonding or metal coordination (Garcia and Fischer, 2014).

Fig. 11.3 shows the general mechanism of intrinsic self-healing. Reversible interac-tions are represented in red and blue colour. When damage occurs, a crack is formed.Intrinsic self-healing is then achieved by the recovery of the former interactions, withor without an external trigger.

Polymer matrix

Polymermatrix

Resinsystem

Hardnersystem

Hollowfiber

CatalystCapsule of

liquid monomer

Crack planeMonomer flows into crack

Contact with catalyst

(a) (b)

Figure 11.1 Microcapsule and hollow-fibre approaches for self-healing (Kessler, 2008).

Figure 11.2 Chemical reaction of a rDA reaction (Wikipedia, 2014).Reproduced from Kessler, M.R., 2008. 22-Self-healing composites. In: Sridharan, S. (Ed.),Delamination Behaviour of Composites. Woodhead Publishing, pp. 650e673; Blaiszik, B.J.,et al., 2010. Self-Healing Polymers and Composites. Annual Review of Materials Research40(1), 179e211.


11.3 Self-healing approaches

This section describes the various extrinsic and intrinsic self-healing techniquescommonly used.

11.3.1 Microcapsules

Capsule-based self-healing is an extrinsic approach of self-healing as the healing agentis embedded in microcapsules. This approach has been widely studied (White et al.,2002; Keller, 2013; Yin et al., 2007).

Microcapsule-based healing consists of embedding a healing agent in microcap-sules that break when cracks appear. The healing agent is then released, and crackrepairing can be achieved. Commonly, a catalyst is used with the healing agent; itacts as a polymerizer in polymer composites. There are four different approaches fol-lowed to use a healing agent and catalyst: (1) The encapsulated liquid agent can becombined with a dispersed catalyst (White et al., 2002), (2) both the healing agentand the catalyst can be embedded in different capsules (Keller, 2013), (3) the healingagent can also directly react with a functionality of the matrix (Yin et al., 2007) underan external stimulus and (d) the healing agent or the catalyst can be placed in the matrixas a separate phase.

Different types of microcapsules have been manufactured and used, usually inepoxy resin matrix composites. Commonly used microcapsules are made of ureaeformaldehyde (Blaiszik et al., 2008; Brown et al., 2003; Brown et al., 2004; Guadagnoet al., 2010; Yang et al., 2011; Coope et al., 2011). Other types of microcapsules, suchas triethylenetetramine (TETA) microcapsules for wear-resistant polymer composites(Khun et al., 2014) and poly(methyl methacrylate) (PMMA) microcapsules with high

Figure 11.3 Intrinsicself-healing approach (Blaisziket al., 2010).


storage and thermal stability (Li et al., 2013), have been manufactured and imple-mented. The size of the microcapsules ranges from 25 to 250 mm. However, someinvestigation has also been conducted on nanocapsules (220 nm) (Blaiszik et al.,2008), and this showed an increase in fracture toughness per volume fraction in com-parison with larger microcapsules.

Several encapsulation techniques have beenused to prepare self-healingmaterials. Theresearch has mainly focussed onmeltable dispersion, in situ and interfacial encapsulationtechniques. Meltable dispersion is the method of dispersing the healing agent in a meltedpolymer to form the capsules after solidification of the polymer (Rule et al., 2005).

In situ and interfacial techniques have been used for ureaeformaldehyde (Blaisziket al., 2008; Brown et al., 2004; Yang et al., 2011) or TETA (Khun et al., 2014) mi-crocapsules. In this technique, the shell is developed by polymerization at the interfaceof healing agent droplets and the oil-in-water emulsion. The synthesis of nanocapsulesusing an ultrasonication technique can also be achieved (Blaiszik et al., 2008). Thegeneral method of healing is demonstrated in Fig. 11.4.

11.3.2 Vascular materials

Self-healing materials that use hollow fibres or a mesoporous network are calledvascular materials (Dry and Sottos, 1993; Williams et al., 2007a,b; Toohey et al.,2009). The approach used is an extrinsic approach and is similar to the microcapsulesapproach, as the healing agent is embedded in fibres or a network of capillaries.

Catalyst

Microcapsule

Healing agent

Polymerizedhealing agent

(i)

(ii)

(iii)

Crack

Figure 11.4 Microcapsule-based self-healing process (Garcia and Fischer, 2014).


The healing agents are released when damage occurs. Healing fibres (i.e., hollow fibreswith a healing agent) have been manufactured to incorporate in fibre-reinforced poly-mer (FRP) composite materials, replacing some of the composite fibres. One of themain advantages of fibres over microcapsules is that fibres can be interconnected toform a network. This allows efficient delivery of the healing agent, and additionallythese fibres can be networked in a way so that a large area can be healed. However,hollow fibres have a greater influence on the mechanical properties of compositesthan microcapsules (Fig. 11.5).

Vascular networks can be one- (1D), two- (2D) or three-dimensional (3D) systems.1D systems were developed by Dry and Sottos (1993) on glass pipettes embedded inepoxy resins. 2D networks are an evolution of 1D systems, and are suitable for theinterface between plies in laminated composites (Williams et al., 2007b). 3D systemsimitate the vascular systems of living beings and are being developed to extend thelifetime of vascular self-healing composites (Toohey et al., 2009).

There are a couple of approaches to build vascular composite materials (Garcia andFischer, 2014). The most common approach is the use of individual hollow fibres,which can replace some reinforcement fibres in FRP composite materials (Pangand Bond, 2005). Hollow fibres can be interconnected, thus forming a vascularnetwork (Toohey et al., 2009). A novel type of hollow fibre called ‘compartmentedfibres’ has been developed by Mookhoek et al. (2012). It has the advantages ofmicrocapsule-based and hollow fibreebased self-healing. Indeed, using this type offibres, a localized healing response can be activated. Another approach similar tothe hollow fibreebased vascular networks is the implementation of a mesoporousnetwork (Coope et al., 2014). Fig. 11.5 shows three different possible approaches ofhollow fibreebased self-healing materials (from left to right): individual hollow fibres,compartmented fibres and interconnected fibres forming a vascular network.

A variety of techniques can be used to manufacture vascular self-healing materials,with the most common approach being the use of hollow glass fibres filled with anappropriate healing agent. For example, hollow glass fibres were filled with a healingagent containing 5% cobalt octate using the capillary action (Zainuddin et al., 2014).

Figure 11.5 Self-healing approaches using hollow fibres (Garcia and Fischer, 2014).


These fibres were then embedded in the matrix by the vacuum-assisted resin transfermoulding (VARTM) process. However, the use of hollow glass fibres is restricted to1D networks. In order to obtain 2D and 3D interconnected networks, steel wires as thinas 0.5 mm could be used (Coope et al., 2014; Norris et al., 2011a,b).

11.3.3 Dissolved thermoplastics

Self-healing materials based on dispersed thermoplastics use an intrinsic, thermallyactivated self-healing approach. The thermoplastic polymer is selected for its goodcompatibility and is dissolved in the polymer matrix, resulting in a homogeneous sys-tem. After damage, healing is triggered by a rise in temperature and pressure so that thethermoplastic healing agent can move and fill the cracks (Hayes et al., 2007).Researchers have tested the properties of a self-healing E-glass fibre epoxy compositecontaining from 5%wt to 20%wt dissolved poly(bisphenol-A-co epichlorohydrin). Ithas the advantage of being suitable for conventional thermosetting composites. How-ever, there is a need for external pressure to heal the cracks, thus limiting its applica-tions in various fields.

11.3.4 Reversible chemical reactions

Self-healing concepts can be accomplished by reversible chemical reactions due to anexternal trigger. The aforementioned type of self-healing concept is called an extrinsicself-healing technique, because an external stimulus such as heat or irradiation isneeded to trigger the healing.

Several reversible chemical reactions have been explored for self-healing applica-tions. The most extensively studied reactions are the rDA reactions. Fig. 11.6 showsthe mechanism of polymerization and repair for DA cross-linked polymers. Chenet al. (2003) demonstrated self-healing properties of furanemaleimide polymers.Bis-maleimide tetrafuran can be used to create a thermally activated self-healingcarbon fibre composite, using a DA reaction and electrical resistive heating of carbonfibres (Park et al., 2010). This composite showed nearly 100% strength recovery undercertain conditions. The DA reaction was used to recover the fibreematrix interfacialshear. This was accomplished by grafting maleimide groups in the carbon fibres that

O

O

O

O

Heat O

O

N

N

Figure 11.6 Mechanism of the DielseAlder (DA) reaction for cross-linked polymers (Parket al., 2010).


can bond with the furans groups in the matrix (Fig. 11.7). Heating was achieved usingresistivity of carbon fibres (Zhang et al., 2014). Other reversible reactions includehydrazone linkages (Deng et al., 2010) and disulphide exchange reactions (Canadellet al., 2011).

The majority of the self-healing composites that act by reversible chemical reac-tions still need an external heat source to start the healing process. Another way toinduce self-healing is by the application of a strong light irradiation (Froimowiczet al., 2011).

11.3.5 Reversible physical interactions

Self-healing behaviours based on reversible physical interactions have been recentlydemonstrated. This involved the use of ionomeric polymers, which are polymers con-taining ionic species, such as metal salts, that can aggregate and form clusters. Owingto the formations of reversible clusters and resulting changes in the mobility of thepolymeric network, they can be used in self-healing applications.

The self-healing response of poly(ethylene-comethacrylic acid) copolymers for bal-listic applications has been researched (Kalista, 2007). Self-healing was triggered bythe impact force. Indeed, clusters relaxations were caused by generated heat. Jameset al. (2014) worked on self-healing piezoelectric ceramicepolymer composites, con-taining zirconium titanate ceramic in a Zinc ionomer ethylene methacrylic acid(EMAA) copolymer matrix. The impact behaviour of an EMAA copolymer with

HNO3

NH2

COOH

COOH

COOHO

O

O

O

O

O O

O

OO

O O

OOO

OO

OO

OO

O

O

O

O

OH

OH

H

N NHNH

NHH

H

H

H

H

N

N

N

N

N

N

N N

NNN

N N

NN

N

NH

NH

NH

NH

HN

NH

NH

C

C

C

C

C

COH

115°C

CF

190°CTEPA

BMI

17 h

80°C 2 h

Figure 11.7 Grafting of maleimide groups on a T700-U carbon fibre surface (Zhang et al.,2014).


acid groups neutralized with sodium ions was studied to implement ionomer compos-ites in spacecrafts for debris protection (Francesconi et al., 2013). The material devel-oped using ionomeric polymer has good self-healing abilities and behaved better thanaluminium plates under very high-velocity impacts.

11.3.6 Reversible supramolecular interactions

Reversible supramolecular interactions are interactions between molecules that aredifferent from covalent bonds or physical interactions, such as electrostatic interac-tions. The interaction includes multiple hydrogen bonds, pep stacking and metalcoordination.

Supramolecular interaction is a promising way to develop intrinsic self-healingcomposites. They are low energy and reversible, and they can have a great influenceon the overall mechanical properties of the material (Fig. 11.8).

A self-healing and thermoreversible rubber from a hydrogen bond supramolecular as-sembly was developed in Cordier et al. (2008). Molecules forming chains throughhydrogen bonding were used. The polymer could be repaired at room temperature andrecovered to its initial form and elasticity. A self-healing material based on both covalentbonds and supramolecular interactions was also developed (Roy et al., 2014). Reversiblesupramolecular interactions were made possible by hydrogen-bonded urea groups, con-nected by a siloxane-based backbone while imine linkages were used as reversible cova-lent bonds. Light-induced self-healing properties for a metallo-supramolecular polymerusing reversible coordination chemistry were also investigated. Like other intrinsic con-cepts, most supramolecular systems need an external trigger to heal, like pressure, light orhigh temperatures.

11.4 Self-healing composite constituent materials

Self-healing properties of materials have been successfully utilized to makeself-healing composites. The first studies on self-healing composites were conductedon the polymer (epoxy resins)ebased composites. Polymers have a good molecularmobility, which explains why the majority of the research about self-healing materialsis still focussed on polymers and polymer composites. However, the interest inself-healing ceramic composites is rapidly growing. Few new concepts ofself-healing have recently emerged, and self-healing MMCs appear to be promisingfor advanced aerospace applications. In this section, different self-healing compositematerials are discussed.

11.4.1 Polymer matrix

Research in polymer-based self-healing composites in the past has been mainlyfocussed on composites made of epoxy matrix (White et al., 2002; Blaiszik et al.,2010; Guadagno et al., 2010; Bond et al., 2008). Recent work in the area has involvedself-healing composite materials, where the composite is made of epoxy vinyl ester,


HOOC

COOH

(CH2)7

HOOC (CH2)7

(CH2)7

COOH(CH2)7

COOH(CH2)7

H15C8 H2N

H2N

H15C8

H13C6

C6H13

NH2

NH2

NH2

NH2NH2

C6H13

O

O

N N N NNH

NH

NH

NH

NH

NH

NH

NH

NHO O O

OO

OO

O

O

1.

2.

Amidoethyl imidazolidone Di(amidoethyl) urea Diamido tetraethyl triurea

Figure 11.8 Soft (H-bond) elastomer synthesis pathway (self-healing) (de Espinosa et al., 2015).

Self-healing

composites

foraerospace

applications343

bismaleimide tetrafuran (2MEP4F), silicones like polydimethylsiloxane (PDMS),cyclopentadiene derivatives or cyanate ester. Raw polymer and diverse compositeslike E-glass fibre-reinforced composites (FRCs) and carbon FRCs were also investi-gated. Raw polymers with embedded microcapsules or hollow fibres can also beconsidered as composites. Composites made of different materials with self-healingbehaviours are discussed in this subsection.

11.4.1.1 Glasseepoxy fibre-reinforced polymer composites

E-glasseepoxy FRCs seem to be the first category of composite materials developedwith healing capabilities, which followed the development of raw self-healing epoxyresins (White et al., 2002; Brown et al., 2003). E-glasseepoxy composites are widelyused because of their light weight, affordable cost and good mechanical properties.Numerous applications for E-glasseepoxy FRC includes boats, sport cars bodiesand aircraft components like radomes.

The self-healing nature of E-glasseepoxy FRCs with a dissolvedpoly(bisphenolA-co-epichlorohydrin) thermoplastic has been investigated (Hayeset al., 2007). The healing phenomenon used was intrinsic, as the thermoplastic poly-mer used was efficiently dispersed in the Araldyte epoxy matrix. A recovery of upto 70% of the virgin material properties was observed, and the observed delaminationarea was also reduced. In another work, self-healing woven glass fabricereinforcedepoxy composite laminates were manufactured, containing epoxy microcapsules asso-ciated with embedded CuBr2(2-MeIm)4 hardener. The composite showedcrack-healing properties after low-velocity impacts (Yin et al., 2008). A temperatureof 140�C was necessary for the healing to occur. Glass fibre composites with anembedded vascular system have been developed and investigated in Norris et al.(2011a,b). The composite was made of 28-ply unidirectional laminates, and glassfibreereinforced epoxy with 0.5 mm vascules. The suitable configuration for vasculeswas found to be aligned in the direction of the glass fibres. Mode I fracture toughnesswas not lowered by the addition of the vascular network between the composite plies.Coope et al. (2014) demonstrated self-healing for a 28-ply E-glasseepoxy unidirec-tional plate by incorporating a series of vascules, parallel to the fibre direction. Theself-healing agent used was a Lewis acidecatalysed epoxy. This composite revealedgood healing efficiency, with good strength and fracture toughness recovery.Embedded hollow glass fibres in a composite made of an SC-15 epoxy matrix rein-forced with a woven (0/90) E-glass fabric has been developed (Zainuddin et al.,2014). The composite was fabricated using VARTM, and the hollow glass fibreswere filled with unsaturated polyester resin using a catalytic method. The authors re-ported a significant improvement in the low-velocity impact properties and the fillingof the cracks by the healing agent after the impact.

11.4.1.2 Carboneepoxy fibre-reinforced polymercomposites (CFRPs)

CFRPs are another category of self-healing composites that have drawn significantattention since the 2000s. Despite their high cost, this type of composite is now widely


used in advanced aerospace applications, due to the high stiffness and strength-to-weight ratios offered by these composites. Some modern civil aircrafts are now builtof more than 50% CFRPs (53% of the Airbus A350XWB is made of CFRPs). Otherapplication areas include automotive and high-end sports goods.

In general, there are fewer studies about carboneepoxy polymer composites thanabout glasseepoxy polymer composites. The initial developments of self-healing car-bon FRCs were noticed in the early 2000s. Kessler et al. (2003) demonstratedself-healing for a structural carbon fibre-reinforced epoxy matrix composite in 2003.The composite was made of a plain-weave carbon fabric in an EPON 828 epoxy ma-trix. In this study, autonomic self-healing was achieved by adding dicyclopentadienehealing agent embedded in poly-ureaeformaldehyde microcapsules by in situ poly-merization. Grubbs’ catalyst (which are transition metal carbene complexes that areused as catalysts for olefin metathesis) was dispersed within the matrix. This compos-ite could recover up to 80% of its virgin interlaminar fracture toughness after delami-nation. Norris et al. (2011b) investigated the best configuration of vascular systems incarbon FRCs. The composite was a laminate, developed of an aerospace-grade unidi-rectional carbon fibreereinforced epoxy preimpregnated tape with a [�45/90/45/0]2Ssequence. The vascular network was implemented between plies using solder wirepreforms. The healing has been found to be efficient when the vascules were locatedbetween the plies. Wang et al. studied a carbon fibreereinforced epoxy matrix compos-ite with poly(ethylene-co-methyl acrylate) (EMA) and poly(ethylene-co-methacrylicacid) (EMAA) patches placed between the plies (Wang et al., 2012). The compositewas made from rectangular prepregs. Results showed an improvement in the interlam-inar fracture toughness, but a decrease in interlaminar shear strength. Additionally,EMAA has been observed to be a good choice for thermoplastic interlayer patches.Hargou et al. (2013) reported a novel ultrasonic welding method to activate EMAAin composites. The fibreematrix interface in a carbon fibreefuran-functionalizedepoxy matrix composite has been investigated (Zhang et al., 2014). Maleimide groupswere grafted to carbon fibres, and the matrix was functionalized with furan groups.Healing was achieved by a thermally activated DA reaction between maleimide andfuran groups. An increase in interfacial shear strength in comparison with a carbonfibreeepoxy system has been observed. Electrical resistive heating was used to activatethe healing process (Park et al., 2010; Zhang et al., 2014).

11.4.1.3 Other self-healing polymer composites

Epoxy matrixebased self-healing composites have been extensively studied since the2000s. But other types of polymers with different matrices and/or reinforcements alsohave been under investigation.

Autonomic self-healing of an epoxy vinyl ester matrix containing microcapsulesfilled with exo-dicyclopentadiene (DCPD), as well as wax-protected Grubbs’ catalystmicrospheres, has been investigated by Wilson et al. (2008). The DCPDeGrubbs’ sys-tem proved to be a good healing agent to fill the cracks in the epoxy vinyl ester matrix.Self-healing was demonstrated to occur rapidly within 2.5 min. Park et al. (2010) syn-thesized a self-healing composite based on a bismaleimide tetrafuran (2MEP4F)


matrix reinforced with carbon fibres, manufactured by a vacuum-assisted injectionmoulding method. The lay-up sequence used for the composite was [0/90/0], andthe fibre volume fraction was 0.46. In this study, healing was based on a thermally acti-vated DA reaction using resistive heating through the carbon fibres. The work demon-strated good recovery of the strain energy after delamination, but the strength wasfound to be much lower than that of the virgin composite. Additionally, self-healingcould not be achieved after the carbon fibres were fractured. A new poly(dimethylsiloxane) (PDMS) self-healing elastomer was developed and investigated by Kelleret al. (2007). The PDMS matrix composite contained encapsulated PDMS resin asso-ciated with a separate cross-linker. A recovery of the virgin tear strength of up to 100%was reported. Furthermore, the incorporation of microspheres increases the tearstrength of the material. Park et al. (2009) fabricated a two-layered composite panelmade of carbon fabric and Mendomer-401, which is a thermally re-mendable cyclo-pentadiene derivative. The self-healing was achieved by electrical resistive heatingvia the carbon fabric. The suitable temperature to trigger the healing was determinedby a dynamic mechanical thermal analysis. Li et al. studied a composite made of anepoxy matrix (EPON 828) with embedded strain-hardened short shape memory poly-urethane (SMPU) fibres and dispersed thermoplastics particles of linear polyester(Li and Zhang, 2013). It was demonstrated that the polymer was able to self-healwide-open cracks initiated by three-point bending tests. The healing was achieved(around at 80�C) by the combination of thermoplastic particles and the shape memoryeffect of SMPU.

11.4.2 Ceramic matrix

Ceramics are widely used for high-end technical applications, because of their highmelting point, excellent stability and good corrosion resistance. However, such mate-rials also undergo brittle failure, as their resistance to crack propagation is very low.Moreover, these materials are particularly sensitive to thermomechanical stresses.To overcome these problems, CMCs have been developed. CMCs consist of ceramicor metal fibres embedded into a ceramic matrix to improve the mechanical properties,such as fracture toughness, tensile strength and elongation at break.

The most common CMCs are made of a carbon, silicon carbide or aluminium oxidematrix, reinforced with carbon, silicon carbide or aluminium fibres. Different types ofCMCs are CeC, CeSiC, SiCeSiC and Al2O3eAl2O3. Carbon fibre-reinforced carbon(CFRC) composites can be manufactured by gas deposition, pyrolysis, chemical reac-tion, sintering and electrophoretic deposition. CFRCs are used to manufacture brakediscs, combustion chamber components, stator vanes, turbine blades and heat shields.

Ceramic matrix materials mainly deteriorate by oxidation. The most effective wayto enhance the lifetime of such materials is, therefore, to stop or reduce oxygen diffu-sion within the matrix cracks by filling them or generating a protective coating. Thematrix composition can be modified to enable the release of a liquid oxide to fill cracksin the case of oxygen diffusion. An example of this is the addition of boron compoundsthat can form B2O4 oxide, which is liquid over 500�C (Lamouroux et al., 1999). In-hibitors like zirconium boride can be incorporated in the matrix to stop or reduce


oxidation of the base material (McKee, 1988). In addition to these methods based onmatrix modification, studies have also focussed on the development of self-healingprotective coatings for CMC materials. Specifically, rare earth silicates have beenused as barrier coatings for SiCeSiC composite materials (Lee et al., 2005).

A SiCeSiC fibre-reinforced ceramic composite with pyrocarbon interfacial coatingsand amorphous B4C contained in the matrix was manufactured by a chemical vapourinfiltration process (Quemard et al., 2007). The samples were predamaged by a loadingbeyond its tensile strength, so as to create cracks in the matrix before exposure to corro-sive environments. In this composite material, self-healing was due to the generation ofB2O3 when oxidation occurs, which was able to seal the cracks, thus preventing theoxygen from further penetrating. It was found that the fibres and interfacial coatingswere well protected by the self-healing, boron-containing matrix. Further work byNualas and Rebillat, (2013) focussed on the oxidation and degradation mechanismsof the B4C phase and the composite material as a whole. They determined the optimumtemperature for self-healing to be 550�C. In another study, Zuo et al. (2012) manufac-tured and oxidized carbon fibreereinforced SiC matrix ceramic composites (CeSiC).SieBeC coatings were implemented in the matrix with the aim of obtainingself-healing behaviour. The composites were shown to have good corrosion resistanceabove 1000�C due to the matrix self-healing properties. The corrosion resistance ofthe SiC modified matrix was induced by the formation of B2O3 oxide, but also by boro-silicate glass that sealed thematrix microcracks. Zhang et al. (2011) worked on the sametype of multilayered CMCs. An improvement in the mechanical properties was estab-lished when the material was exposed to high temperatures, highlighting self-healingbehaviour.Mohanty et al. fabricatedAl2O3eSiC ceramic composites containing yttriumoxide (Y2O3). Microcracks were generated in the matrix using an indentation method(Mohanty et al., 2011). The self-healing property was demonstrated and was reportedto be the consequence of SiC oxidation when oxygen penetrated into the cracks. Theoxidation occurred under high temperatures and in the presence of additives, like Y2O3.

11.4.3 Metal matrix

MMCs are advanced composite materials that consist of a metallic matrix reinforcedwith different materials. MMCs have good mechanical properties like strength, stiff-ness and modulus of elasticity, along with relatively low density. However, thesecomposites are generally much more expensive than traditional alloys and polymermatrix composites. As a consequence, their use is limited to a number of high-techapplications, such as aircraft structures, space systems and high-end sportsequipment. Specifically, MMCs can be found in disc brake and landing gearcomponents.

In most structural applications, the matrix is made up of a light metal or alloys likealuminium, magnesium or titanium. Reinforcements are usually carbon, boron or sil-icon carbide fibres. Particles of alumina and silicon carbide can be used so as to obtainisotropic properties.

Fig. 11.9 shows three different ways of achieving self-healing in MMCs. The useof a healing agent, containing capsules and shape memory alloy (SMA) wires, has


been investigated. Another possible approach is the synthesis of an oversaturatedsolid solution as a matrix. Atoms fill the cracks when damage occurs due to themetastable equilibrium of the solution.

Self-healing MMCs have recently been developed, although the extent of researchremains far less than that for polymer matrix composites and CMCs. The studies havegenerally focussed on the feasibility of different approaches for implementingself-healing properties. Different concepts have been investigated in recent years,including the use of SMA fibres, low-melting point alloy containing hollow reinforce-ments, a supersaturated solid solution and a eutectic phase embedded in a dendriticstructure.

Embedded healing agents, containing hollow reinforcements, can be used todevelop a self-healing composite. Specifically, an aluminium alloy reinforced by sol-der alloy, containing alumina microtubes, was developed (Lucci et al., 2009). Similarto the polymer systems, the healing agents flow and fill the cracks when hollow fibresare damaged by crack propagation. The use of SMA wires in a solder matrix wasassessed by Manuel (Manuel, 2007). In this study, TiNi SMA wires were embeddedin a Sn-based alloy matrix containing Mg (5.7 at%) and Zn (2.7 at%) in order to obtaina high-strength self-healing MMC. The study revealed that the composite recovered94% of its virgin tensile strength after damage, and the uniform ductility increasedby 160%. To obtain greater strength and healing efficiency, the addition of a eutecticalloy with a dendritic structure at a low melting point was investigated (Nosonovskyand Rohatgi, 2012; Ruzek, 2009). The eutectic phase would melt and heal the crackswhen heated, and the dendritic phase would remain solid during the healing process.

Crack

Catalyst

Fracturedcapsule

Capsule with a healing liquid

Grainboundary

Solute atomsVoid

Crack

Shape-memory alloy

(a) (b)

(c)δx

Figure 11.9 Three self-healing approaches for metallic composites (Nosonovsky and Rohatgi,2012).


The combination of SMA wires with a metal matrix could be useful for novelself-healing metal alloys. The use of an off-eutectic phase has also been investigated(Ruzek, 2009). Off-eutectic systems had the ability to exist in solid and liquid states atthe same time in equilibrium conditions. The self-healing approach was the same aseutectic and dendritic systems. The goal was to create a system that can partly meltand heal the cracks while a solid phase maintains the structural integrity of the com-posite. Another thermodynamic solution to create a self-healing metal composite isthe use of supersaturated solid solutions as a matrix (He et al., 2010). In this concept,fatigue or damage-induced microcracks act as nucleation points for the supersaturatedalloy. Cracks are therefore filled and healed when precipitation occurs naturally.

In summary, the possibilities of metal matrix self-healing composites appear to bepromising, and a number of concepts have been explored to achieve efficientself-healing.

11.5 Functionality recovery in self-healing composites

Extension of service life and easy maintenance of composite structures are the mainreasons behind developing self-healing technologies. Indeed, such materials are ableto self-repair in case of damage, thus sensibly improving their lifespan. External inter-vention is not even required in the case of autonomic self-healing materials. Butself-healing cannot be perfect, and damage or ageing always leads to a partial lossof the composite mechanical properties. Hence, it is important to estimate the healingefficiency for each material and each self-healing concept. Moreover, it is accepted thatonly a limited number of repeated damageehealing cycles can be handled byself-healing materials. Even though the properties that can be recovered byself-healing materials are diverse, healing efficiency can as defined as (Blaisziket al., 2010):

h ¼ f healed� f damagedf virgin� f damaged

[11.1]

where f is the property of interest.Table 11.1 shows several healing efficiency data obtained by researchers for studies

on different composite materials with different healing approaches. The properties ofinterest and the loading conditions are very diverse. Healing efficiency can exceed100% because healed properties may be better than virgin properties in some cases.Additionally, Fig. 11.10 shows a comparison between pre- and posthealing of a micro-crack in Mendomer40ecarbon fibres (Park et al., 2009).

11.5.1 Structural integrity recovery

Research on self-healing composite materials mainly has been aimed at recoveringstructural integrity, rather than focussing on mechanical properties (White et al.,2002). Indeed, loss of structural integrity under service loading is a major threat to


Table 11.1 Self-healing efficiency of some composite materials (Blaiszik et al., 2010)

MaterialHealingapproach Loading conditions Property of interest

Healingefficiency (%) References

Mendomer401ecarbonfibres

Reversible DAreaction

Three-point bending Strain energy 94 Park et al. (2009)

2MEP4F polymer Reversible DAreaction

Compact tension Fracture toughness 83 Chen et al. (2003)

EpoxyeE-glass fibres Microcapsules Double-cantilever beam Fracture toughness 60 Kessler andWhite (2001)

EpoxyeSMA wires Microcapsules andSMA wires

Tapered double-cantileverbeam

Fracture toughness 77 Kirkby et al.(2009)

Epoxy vinyl ester Microcapsules Tapered double-cantileverbeam

Fracture toughness 30 Wilson et al.(2008)

Epoxyecarbon fibres Microcapsules Width-tapereddouble-cantilever beam

Fracture toughness 80 Kessler et al.(2003)

EpoxyePCL phase Meltable phase Single-edge notched beam Peak fracture load >100 Keller et al.(2007)

PDMS Microcapsules Tear test Tear strength >100 Luo et al. (2009)

350Advanced

Com

positeMaterials

forAerospace

Engineering

composites, since they are made up of different materials that respond differently tothermomechanical loadings. In particular, composite materials are subjected to matrixmicrocracking, delamination and fibre de-bonding. A number of studies were carriedout to assess the healing efficiency on such structural damage.

Matrix microcracking can lead to wide-open cracks and then material failure.Numerous studies have been carried out to assess the efficiency of differentself-healing approaches for healing the cracks. DCPD-containing microcapsuleswere notably used to repair fatigue cracks in epoxy matrix composites (Brownet al., 2005). As a consequence, composite fatigue life was extended by 118e213%in this research. Healing of cracks was also observed for C/SieBeC CMCs (Zuoet al., 2012). Cracks were found to be efficiently sealed by a borosilicate glass, thusproviding the composite with good resistance to oxidation. In another study, dispersedthermoplastic particles were used to heal wide-open cracks (Li and Zhang, 2013). Thisself-healing process is called close-then-heal, because wide-open cracks are closed

60 μm

60 μm

(a)

(b)

Figure 11.10 Configuration of a crack (a) before and (b) after healing (Park et al., 2009).


using the shape memory effect of SMPU fibres before healing. The cracks were foundto be narrowed by the SMPU fibres and healed effectively. Delamination is anotherweakness of composites subjected to thermomechanical stresses. Recovery of struc-tural integrity after delamination damage is a serious concern. A number of researchstudies have been carried out to assess the efficiency of healing on delaminationdamage. The concepts include the use of dissolved EMAA thermoplastic agent(Pingkarawat et al., 2012), a microvascular network (Trask et al., 2014) and mendableEMAA filaments stitching (Yang et al., 2012). Along with delamination damage, fibrede-bonding is also a potential threat to structural integrity for fibre-reinforced compos-ite materials. Some studies have focussed on preventing or repairing such damage(Blaiszik et al., 2010; Zhang et al., 2014; Carr�ere and Lamon, 2003).

11.5.2 Mechanical properties recovery

Mechanical properties are the most important properties for aerospace structural ma-terials. As a consequence, it is crucial to know the exact healing efficiency of aself-healing material from the point of view of using it for aerospace applications.Most studies about self-healing materials have focussed on mechanical properties re-covery as a way of assessing self-healing efficiency; this is particularly true for fracturetoughness recovery. Researchers have also assessed impact strength and fatigue resis-tance recovery.

11.5.2.1 Fracture property

Fracture toughness recovery can be evaluated through different mechanical tests, such asthree-point bend, compact tension, double-cantilever beam, tapered double-cantileverbeamandwidth-tapered double-cantilever beam tests.Depending on the self-healing con-cepts, the fracture toughness recovery rate may range from 30 to 100%.

A recovery rate of 75% of the fracture load of a microcapsules-containing epoxyresin has been observed (White et al., 2002). However, most researchers prefer fracturetoughness recovery rather than peak fracture load recovery to analyse healing effi-ciency. The microcapsule-based healing technique has been researched and analysedsince the 2000s (Mangun et al., 2010; Yuan et al., 2011a,b; Jin et al., 2012; Tripathiet al., 2014). It was demonstrated that fracture toughness recovery was highly depen-dent on the quantities of healing agent and catalysts utilized. Reversible DA reactionswere effectively used in CFRP composites and 2MEP4F polymer systems (Park et al.,2009; Chen et al., 2003). These crosslink polymeric materials proved to heal effi-ciently, with recovery rates exceeding 80%.

11.5.2.2 Impact property

Impacts present critical hazards to aerospace composite structures, as impact loads candamage composite panels, which can be difficult to detect. Heavy maintenance proce-dures and nondestructive control methods have been developed to address these issues,but the use of self-healing composite materials could potentially be a simple and


efficient solution. A number of researchers have focussed on damage recovery afterimpact loading and impact properties recovery after impact. Low velocity, high veloc-ity and repeated impacts were investigated (Zainuddin et al., 2014; Francesconi et al.,2013; Yuan et al., 2011b; Chunlin et al., 2013; Nji and Li, 2012).

Glass fibreereinforced epoxy laminates, with embedded microcapsules, wereexposed to low-velocity impact (Yuan et al., 2011b). Healing efficiency was investi-gated using a scanning electron microscope to calculate the reduction in damagedareas. It was shown that matrix microcracks were completely healed, and the structuraldamage was significantly reduced. Under low-velocity impacts, the efficiency of ahealing agent, containing hollow fibres embedded in E-glasseepoxy composites,was studied (Zainuddin et al., 2014). Improvements in peak load (53%) and energyto peak load (86%) after a second impact were observed. Investigations were also car-ried out on high-velocity impacts. A study was carried out to identify whether aself-healing ionomeric polymer could possibly replace aluminium panels for spacedebris protection (Francesconi et al., 2013). It was found that damage recovery wassatisfactory. Conversely, aluminium was found to have a slightly better fragmentationbehaviour. Chunlin et al. (2013) manufactured self-healing sandwich composite lam-inates with an embedded vascular network. Specific stiffness showed good recoveryafter impact damage, but the composite’s skin strength recovery still needed to beimproved. The recovery of properties after repeated impact loading was also examined(Nji and Li, 2012). A thermoplastic composite containing shape memory polymer ma-trix reinforced with a 3D woven glass fibre fabric has been investigated. An automatedhealing at room temperature was observed after low-velocity impacts (Yuan et al.,2011b).

11.5.2.3 Fatigue property

Although polymers are not extensively used for fatigue-loading applications comparedto metallic materials, structural composite materials may exhibit fatigue cracks due tocyclic mechanical loadings. Self-healing materials could be a promising solution tothese cases, because these materials can heal cracks, including fatigue cracks, thusextending fatigue life.

The healing of fatigue damage in vascular epoxy composites has been investigatedin Hamilton et al. (2012). The material was exposed to high levels of stress, up to 80%of the quasistatic fracture toughness. However, healing was demonstrated to be mostefficient for lower loads, because the healing agent could efficiently heal the fatiguecracks. In these conditions, the rate of the crack extension was reduced by morethan 80%. Neuser and Michaud (2014) focussed on the fatigue response of an epoxymatrix composite with embedded EPA-containing microcapsules and reinforced bySMAwires. The operating healing mechanism was unique to fatigue loading. EPA sol-vent arrested crack growth when released, and the composite toughness was improved.Additionally, SMA wires resulted in healing and recovering the structural integrity. Inaddition to experimental studies, some investigations were aimed at understanding thefatigue life of self-healing composites using phenomenological and physical models(Jones and Dutta, 2010).


11.5.3 Barrier and corrosion protection recovery

Corrosion resistance is a major concern for aircraft parts and especially for thoseexposed to external conditions. Specifically, metal alloys and polymer compositesare used in fuselage parts. Moreover, engine blades and combustion chambers aresubjected to very high temperatures and corrosive atmospheres, compounding theissue. A number of studies have focussed on recovering the barrier and corrosion pro-tection properties of both coatings and bulk composite materials using self-healingconcepts.

Self-healing coatings aimed at protecting aerospace-grade aluminium alloys fromcorrosion have been researched (García et al., 2011). The coating is based on silyl es-ters that react when corrosion damage occurs. Reversible disulphide links were used torecover mechanical properties of a damaged surface (Canadell et al., 2011). Mechan-ical properties were shown to fully recover, even after numerous healing cycles. Disul-phide chemistry could be an advantage to build smart, self-healing polymer coatings.In another study, photo-induced healing was achieved for a dendritic macromonomer,and full cross-links reversibility was demonstrated (Froimowicz et al., 2011). Thiswork paves the way towards developing self-healing aerospace coatings that couldbe triggered by sunlight, with no need of other external intervention. The use of micro-capsules containing coatings was assessed on steel substrates (Yang et al., 2011).Epoxy-filled ureaeformaldehyde microcapsules were found to efficiently protect theunderlying substrate as the treated samples showed no sign of corrosion, contrary tothe control steel samples. TiO2-containing polymer composite coatings were alsoinvestigated (Yabuki et al., 2011). This concept was found to significantly increasethe corrosion resistance.

11.6 Applications of self-healing composites

11.6.1 Aerospace applications

11.6.1.1 Engines

Conventional ceramic composites are now widely used in jet engines because of theirexcellent thermal resistance. However, sensitivity to brittle failure and impact damagereduce ceramic parts’ lifespan and reliability. In addition, it is nearly impossible to useceramic composites in mobile parts, which can be exposed to impacts, such as turbineblades. These parts are usually manufactured with nickel superalloys. But the meltingpoint of nickel prevents engine manufacturers from being able to increase workingtemperatures, which limits an engine’s efficiency. Hence, a number of studies havebeen carried out recently to assess self-healing CMCs as solutions for fixed and mobilejet engine parts.

Replacing existing ceramic composites with self-healing composites in enginecombustion chambers has been attempted. Multilayered, boron-containing matrixcomposites were investigated as alternatives. The oxidation behaviour of the fibresand the matrix under relatively low temperatures for a SiC(f)/PyC(i)/SieBeC(m)composite was described (Nualas and Rebillat, 2013). In self-healing ceramic


composites, the healing behaviour was due to the formation of boron oxide (B2O3) thatcan seal the matrix cracks. Investigations were also carried out under higher tempera-tures of 1000�C, 1200�C and 1350�C, which can typically be found in an aircraftengine combustion chamber (Liu et al., 2011).

The composite was made of a 2D-C/[SiCe(BeC)] ceramic containing boron and aboron silicon glass phase. Self-healing was achieved when the boron compounds andboron silicon glass were oxidized, which led to the formation of phases that could flowin the matrix cracks and seal them. It was found that the three-point bending strengthand tensile strength increased with temperature. These results showed that the specificceramic composite displayed a self-healing behaviour, and that it could be suited foruse in aircraft combustion chambers. The use of boron was further investigated forinterfacial pyrocarbon, in addition to the boron-doped matrix, to enhanceself-healing properties and corrosion resistance (Fig. 11.11) (Naslain et al., 2004).In particular, a SiCeSiC fibre-reinforced ceramic composite material with a multilay-ered matrix containing B-doped pyrocarbon layers and B4C and SiC layers wasassessed. Boron-bearing species in the interphase layers and in the matrix itself (pyro-carbon and B4C) give the composite a self-healing ability resulting from the formationof fluid B2O3 under oxidizing conditions. This type of composite is reported to haveexcellent corrosion resistance under mechanical loads and high temperatures andseems to be suitable for jet engine combustion chambers. Osada et al. (2014) discussedthe design of self-healing ceramic composite turbine blades for jet engines.Self-healing technology would allow the use of ceramic turbine blades instead ofnickel superalloy turbine blades. Indeed, current ceramic composites are sensitive toimpact under external loads that can lead to blade failure. The use of ceramic bladeswould enable higher working temperatures, thus improving turbines’ efficiency. Theresearchers explored nanocomposites’ and multicomposites’ self-healing approaches.It was demonstrated that such composites could effectively self-heal under high tem-perature and low oxygen partial pressure.

10 μm

Figure 11.11 Self-healing com-posite showing matrix depositionaround the fibres after healing(Naslain et al., 2004).


11.6.1.2 Fuselage

FRP composites are now widely used for aircraft fuselage, and the latest civil aircrafts(Boeing 787 and Airbus A350) are developed with up to 50% FRP composites byweight. However, current FRP composites are susceptible to damage under impacts,which often leads to heavier designs to meet safety requirements. Additionally, exten-sive maintenance is needed as impact damage on composites is difficult to detect andassess. Self-healing is one of the most promising approaches to overcome impact loadvulnerability and to design low-maintenance, lightweight composite fuselages.

Studies were carried out on self-healing hollow glass fibreeepoxy composites foraerospace structural applications. In particular, a composite prepared by mouldingwas investigated (Tan et al., 2008). The specimens were damaged by three-pointbend impact tests. Postdamage flexural tests showed a strength recovery of up to47% after healing. The implementation of simple self-healing composites could there-fore dramatically improve the resistance of aircraft composite structures to impactdamage. However, most aerospace-grade composites are plastics reinforced with car-bon fibres. Hence, many studies have focussed on assessing the impact and mechan-ical properties of carboneepoxy self-healing composites (Bond et al., 2008; Williamset al., 2007b). In particular, Bond et al. synthesized self-healing carbon fibreereinforced epoxy laminates to overcome the generally poor impact resistance offibre-reinforced plastics. Two different studies about the self-healing efficiency ofthese composites were conducted. Initially, the flexural strength of a damaged16-ply aerospace-grade composite with embedded 70 and 200 mm hollow glass fibres(HGFs) was investigated (�45�/90�/45�/0�/HGF/�45�/90�/45�/0�/0�/45�/90�/�45�/HGF/0�/45�/90�/�45�). The results showed a 97% strength recovery after quasistaticindentation damage. The compressive strength after impact on the same compositereported an average recovery of 92% after impact damage and healing. The resultsrevealed that the FRP composites with the healing capability can be extensivelyand efficiently used in aerospace fuselages as HGF self-healing composites. Toimprove the impact properties of self-healing polymer composites for aerospaceapplications, the use of superelastic shape memory NiTi alloy wires in compositeswith glass fabrics or carbon fabrics was investigated. The glass fabric compositelaminate was a glass fibreereinforced vinyl ester resin with a stacking sequence of[0�, W, 90�, W, 0�, 90�, 0�]s. The carbon composite was a vinylester resin reinforcedby a carbon fabric with a stacking sequence of [0�, W, 90�, W, 0�, 90�]s. SMA-containing composites revealed to have a significantly higher Charpy impact energyin comparison with standard composites. However, damage tolerance of carbon com-posites under repeated impacts was reduced by the addition of SMA wires. So the useof SMA wires could be a promising way to render better impact resistance to aero-space glass fibre composites, but not necessarily carbon-reinforced composites.Self-healing polymer composites were also assessed as an alternative to metal alloysfor protecting space structures from space debris (Francesconi et al., 2013). Particu-larly, an EMAA copolymer with acid groups neutralized with sodium ions wasprepared. The ionomeric polymer showed excellent self-healing abilities underhigh-velocity impact. Additionally, polymer plates were found to be more


impact-resistant compared to typical aluminium alloy plates, although debris frag-mentation properties were slightly better for aluminium plates. As a consequence,ionomeric copolymers could be used for space structure composites, in addition tomaterials with good fragmentation abilities. In particular, the self-healing ability inspacecraft applications could be of great importance for critical pressurized modulesand reservoir-type components.

11.6.1.3 Aerostructures

As FRCs are being increasingly incorporated in aircraft structural parts, it is likely thatself-healing composites will be used for such applications. Indeed, self-healing com-posites have a greater resistance to fatigue, because they can heal microcracks beforeany crack growth or extension leads to failure. Moreover, their virgin mechanical prop-erties are sometimes higher than those of conventional composites.

A study was conducted to assess the effects of the healing components on the me-chanical properties of composites to determine their use in aerospace structural appli-cations (Guadagno et al., 2010). The studied composite was made of a DGEBA epoxymatrix with embedded DCPD-containing ureaeformaldehyde microcapsules. Thecomposite’s elastic modulus was found to slightly decrease due to the addition ofthe catalyst in the matrix. However, a good recovery of the virgin mechanical proper-ties after healing was evident. Most aerospace structural composites are epoxy com-posites. Teoh et al. (2010) carried out research on a self-healing epoxy compositewith an embedded healing agent, containing HGFs. The specimens were damagedby indentation and then underwent three-point bend flexural tests. Results showedgood strength recovery after healing. The investigation confirmed that self-healingHGF polymer composites could have a strong potential as future materials for aircraftstructural parts. Other tests focussed on E-glasseepoxy aerospace-grade composites(Coope et al., 2014; Norris et al., 2011a). Coope et al. demonstrated self-healingfor an aerospace-grade E-glasseepoxy plate by incorporating a series of vascular net-works parallel to the fibres’ direction. As explained in the E-glasseepoxy FRCs sec-tion, the self-healing agent was a Lewis acidecatalysed epoxy (Coope et al., 2014).The composite was found to fully recover its virgin mechanical properties. Conse-quently, self-healing glasseepoxy composites can be effective substitutes for aero-space fibre-reinforced polymer composites, as they have the potential to overcomecurrent limitations. Similarly, Norris et al. (2011a) developed a glass fibre compositeusing an embedded vascular network to convey healing agents for incorporatingself-healing FRP laminates in primary aircraft structures. The results also revealedthat the addition of vascules had an insignificant effect on the fracture toughness. Asa consequence, vascular self-healing FRP laminates could be a reliable solution toreplace existing laminates in aerospace structural applications, besides facilitatingmaintenance and reducing weight through healing. Carbon fibreereinforced polymercomposites are also used widely for structural applications, especially in military andrecent civil aircrafts (Airbus A350XWB and Boeing 787 Dreamliner). A carbon fibreereinforced polymer matrix composite, combining a shape memory polymer with carbonnanotubes and a (0�/90�/90�/0�) carbon fabric, was manufactured using a high-pressure


moulding process (Liu et al., 2013). The self-healing behaviour was exhibited by thepolymer’s reversible cross-links that rendered the system’s shape memory ability.The composite was found to efficiently heal matrix cracks and regain its structuralintegrity after the initiation of damage under tension. Additionally, up to 72% of thevirgin mechanical properties (peak load) were recovered after healing. In summary,self-healing polymer composites could replace some existing aerospace structuralcomposites, thus enhancing aircraft damage tolerance, service life and safety.

11.6.1.4 Coatings

Coatings and paints are of crucial importance in the aerospace industry, because theyprotect fuselage and structural parts from external and environmental conditions thatcan induce corrosion damage. Self-healing coatings appear to be promising as theywould automatically recover their protection ability after any damage. Self-healingcoatings for protecting aerospace structures from corrosion and minor impacts canbe realized with self-healing epoxy resin composites (Yang et al., 2011). Coatingsare less critical than structural parts, because their failure usually does not lead tothe entire structural failure. Yang et al. (2011) synthesized an epoxy resin compositecoating for steel alloys with embedded ureaeformaldehyde microcapsules. Microcap-sules proved to be suitable for the paint application of the epoxy resin composite asthey did not break apart during this step. In addition, the coated steel samples were suc-cessfully protected against corrosion even after the coating was damaged and healed.

Coatings for metallic alloys are of importance because aluminium, titanium andmagnesium alloys are key materials for both civil and military aircraft and needcorrosion protection. Hamdy et al. focussed on self-healing vanadia coatings foraerospace-grade aluminium (Hamdy et al., 2011a,b) and magnesium (Hamdy et al.,2011b; Hamdy and Butt, 2013) alloys, The coating was synthesized by chemical con-version on an AA2024 aluminium alloy. Vanadia coating was found to be satisfactory,especially when prepared from a 10 g/L vanadia solution. Self-healing properties areexplained by the formation of a thin vanadia oxide film that prevents oxygen fromfurther penetrating. Vanadia-containing coatings could be a good alternative tochromate coatings for aerospace-grade aluminium and magnesium alloys.

11.6.2 Other applications

Beyond aerospace structures, engines and coatings applications, the development ofself-healing composites opens up a vast field of applications in automotive,high-end sports goods and a variety of industries. However, due to their high costand complexity in manufacturing, self-healing composite materials are likely to beused only for high-tech, critical applications, such as aerospace, the nuclear industryand electronic components.

Much research has been carried out on self-healing ceramics for aircraft engines,but the use of such composites has also been investigated for solid oxide fuel cells, nu-clear applications and hot coal combustion conditions (Rebillat, 2014). Self-healing


ceramics could replace existing ceramics because they are less sensitive to brittle fail-ure and have a longer lifespan; however, the cost is a limiting factor.

Predictions of the behaviour of MMCs are complex, because of the expensive testsand cost of production. Moreover, self-healing concepts in metal are relativelynew and need more investigations for practical use. However, research has led tosome potential opportunities, notably for more sustainable electronic components(Nosonovsky and Rohatgi, 2012).

Self-healing polymer matrix composites can be manufactured easily and are lessexpensive, compared to self-healing ceramics and metals. Self-healing polymer com-posites are finding various applications. Notably, a number of organic coatings andpaints have been investigated involving self-healing, polymer composites (Garcíaet al., 2011). Additionally, coatings for protecting ships and offshore oil platformsfrom seawater environments also have been researched (Yabuki et al., 2011). The ef-ficiency of self-healing polymer composites has been evaluated for improving the life-time of organic light-emitting diodes (OLEDs) (Lafont et al., 2012).

11.7 Summary

Self-healing composite materials have been investigated extensively since the 2000s.Numerous concepts have proved to be remarkably efficient in polymer, ceramic andmetal matrix composites. These developments could pave the way to several applica-tions, specifically in the field of aerospace. Self-healing can be achieved using healingagents containing microcapsules, vascular networks, dissolved thermoplastics andreversible interactions in polymer matrix composites. Self-healing abilities were alsodemonstrated for CMCs and MMCs. Self-healing composites are likely to be animportant component in aerospace applications, particularly in addressing fatigueand impact resistance problems. Corrosion and barrier properties can also be efficientlyrecovered after healing. Applications in the aerospace sector include fuselage and aero-structures, engine blades, combustion chambers, anticorrosion coatings, smart paintsand impact-resistant space structures.

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